I don't know whether I can put my opinion here, but yet...
I don't think that mixing styles is so bad idea. Yes, maybe it isn't good when working in a team, but if you work on a code on your own and it's convenient for you - why not?
For example, I prefer the OO style generally. But when it comes to setting connection, the procedural style provides opportunity to use easy 'or' syntax:
<?php
$connection = @mysqli_connect('localhost', 'root', '1234', 'db') or die('Connect Error (' . mysqli_connect_errno() . ') ' . mysqli_connect_error());
?>
which is more convenient for me than:
<?php
$connection = @new mysqli('localhost', 'root', '1234', 'db');
if ($conn->connect_errno) {
die('Connect Error (' . $connection->connect_errno() . ') ' . $connection->connect_error());
}
?>
So why shouldn't I use the procedural style when setting connection?